today at 8:22 PM
The worst thing about consumer (AM5) Zen4 and Zen5 is the I/O die chiplet and memory controller on it. I've heard they're finally going to ship a new I/O chiplet in Zen6 on a much smaller node. Their existing AM5 Zen4/5 memory controller when using both memory channels is terrible. Extremely hard or impossible to get above 3600 MT/s.
Of all the things I'm looking forward to on Zen6, an improved memory controller is at the top of the list.
Edit: and yes, others may say the paltry few PCIE 5.0 lanes is the worst thing about AM5, but that doesn't affect my usage much, and unfortunately I think that's a limitation of AM5 itself rather than the I/O chiplet.
today at 10:12 PM
It's not too hard to get above 3600 MT/s (I assume what you actually want is to get the uclk above 1800) if you bump the soc voltage, but at default voltages there's no chance. Most people overclocking don't touch the soc rail. I've done it on a few zen4 parts and supposedly there are golden samples that go above 1800 without major soc voltage increases but I've never seen evidence of one.
today at 9:04 PM
Agreed, this is likely going to change the landscape for people ekeing everything out of an AMD-based HEDT.
To take full advantage of it, folks likely will want A) a new motherboard with likely a new chipset to support the higher frequencies and B) <GULP> higher frequency DDR5.
I’d love more memory bandwidth with the limited 2-channels I have, but the 2x48GB (96GB) of “tuner memory” I bought in February 2025 I’m not even sure is manufactured anymore. I paid $440 back then, and my understanding is it’s not produced now, and commands $2500 - $3000 on the second-hand market.
today at 8:59 PM
(AM4 was 3600 MT/s; AM5 is 6000 MT/s.)
today at 9:03 PM
Reported even on AM5, you can only get 6000 Mt/s with 2 sticks, attempting to run all 4 slots on most boards won't work unless you slow them down to base JEDEC speeds.
today at 10:01 PM
The brach predictor it's the bane of programmers. It forces to waste effort to write unreadable branchless code.
X86 should support no-branch-predict-if and dont-predict-goto
today at 10:11 PM
You've got it the wrong way around. If the branch predictor was 100% accurate, branchless code would have no performance benefit compared to branching code.
> X86 should support no-branch-predict-if
It almost does. You can use __builtin_expect to hint if the branch is predictable - like assertions. Don't annotate conditionals if they're truly unpredictable. The branch predictor will run, but it's not expensive.
today at 10:10 PM
Won't that just stall the pipeline on a modern superscalar core and cause even worse performance?
today at 5:53 PM
The instruction level parallelism in the 2-ahead branch prediction is super cool stuff, I know zen5 had this feature but it's kind of crazy that memory is so slow relative to the cores that this complexity is beneficial and worthwhile.
today at 8:38 PM
The "compute" part of the CPU is almost negligible, most of it is SRAM, reordering and branch prediction
today at 9:22 PM
SIMD stuff is also generally pretty big part of the chip, tho of course dwarfed by cache
today at 6:24 PM
> AMD has reduced the number of integer multiply units from three to two, with PDEP and PEXT throughput also falling from three instructions per cycle to two.
Are PDEP and PEXT using the same (parts of) circuits as multiply (they are very different operations...), or are this just two independent drops of circuitry and the wording is just a bit unclear?
today at 7:36 PM
They are unlikely to share circuits with the multipliers, but the 2 of the 6 execution units that include multipliers must be identical, to simplify layout, and the PDEP/PEXT circuits are allocated to these 2 units, like the multipliers.
today at 9:29 PM
IIRC, PDEP/PEXT are generalizations of shifter networks.
today at 6:38 PM
32 cores is a lot of cores in a CCD. Is there a there any smaller logical unit between a CCD and a core, or are you expected to wrangle 32 cores in your lowest NUMA level?
today at 6:57 PM
From what I've seen there is no intermediate unit. IMO this is good because it allows larger VMs that don't have to straddle L3s.
today at 7:42 PM
32 cores is only on the Zen 6c CCDs. There are 12 cores on the Zen 6 CCDs. Also not all 6c parts have the full 32 cores per CCD
today at 6:23 PM
What looks like a lot of small balanced improvements to the core itself. Four vector loads per cycle at 256b of less, I wonder if the gather performance is finally acceptable? Another balance example, they actually reduced the number of integer multipliers down to two. I wonder if that was to make diespace for the vnni pipes?
The real star looks like the c cores though. 32 C cores per chiplet with increased l3 cache is impressive
today at 7:41 PM
today at 7:03 PM
Seems like they are starting to optimize the C core architecture already. Wonder how long it'll be before it becomes its own line up and branding.
today at 7:21 PM
This is amusing since Intel seems to be sort of going in the other direction lately: their efficiency cores are becoming more like their performance cores with each new generation (at least in so far as they are getting wider and more capable of higher clock speeds), and there even seem to be some rumors that the two design trains may get "merged" in the future.
today at 5:23 PM
One thing I wonder about is that these CPUs are just so much faster than their memory controllers. So why not add more? Why don't they have a memory controller per core, for example?
today at 9:47 PM
Memory controllers are pretty much limited by external pinout.
Zen 4 and newer consumer chips have 4 memory controllers (for legacy reasons shown as 2) which handle 32bit wide LPDDR5 or DDR5 interfaces (2x per dimm for DDR5)
The AM5 (desktop) socket has in addition to the 4x 32bit memory lanes, 28 PCI-E lanes (generally 16x for GPU, two 4x general purpose, one 4x to chipset), 4x combined DP/HDMI outputs, USB 2.0 port, 4 USB 3.2 Gen 2 ports. And that's just the major buses, there's ton of other components exposed (simpler IO, sound, power)
All of that counts to staggering 1718 pins - anyone remember when moving dual-channel K8 from 940 pin to 939 allowed removal of whole layer from motherboard PCB, making them way cheaper?
The high-end performance server socket from AMD has 6096 pins, significant chunk of it being the 24x 32bit DDR5 interfaces.
So unfortunately it's very much not easy to add more memory, and that's before considering issues with routing it on motherboard even if you accept changing shapes
today at 9:59 PM
I have no real idea of how much a "pin" costs though; it seems There was a brief period of affordable threadripper, but then product segmentation made that not great for amd...
Perhaps it's better to stick on am5 for a while, but it's not clear if this number of pins is somehow cost effective, or if it's just enough to beat Intel.
today at 5:34 PM
Cost, and especially now, cost of memory! But you can buy AMD chips with 2 controllers (consumer), 4 (threadripper), 6 (8005), 12 (EPYC) memory channels, so in effect you can. Next generation Zen 6 Epyc has 16 channels. Roughly you get about 1 channel per chiplet, but you csn adjust if you need more memory bandwidth by buying fewer chiplets, eg there is an 8 core EPYC if you want to optimisise for bandwidth. But many applications are not bandwidth bound so this isn't the right choice, eg they are IO bound, rarely compute bound.
today at 5:30 PM
I think it mostly is for more controllers to be useful that means more PHYs and more actual pins on the package and more traces on the board and more sockets for the memory and... The space and cost of all of that is substantial (to a point, and then it just physically can't be manufactured).
today at 5:38 PM
More memory controllers would add more bandwidth. That can help, but few workloads are bottlenecked by memory bandwidth. (Dual memory channels have been a thing in desktops and laptops since the early 2000s single-core era at least, but newer generations of DDR have increased bandwidth to keep up with increased core counts.) What's really needed is less memory latency (less latency = faster), which has not improved as much as bandwidth.
today at 6:49 PM
Strata is!
today at 6:21 PM
That doesn't help memory latency and most of the time a cpu is waiting it's because of latency, not bandwidth.
today at 5:59 PM
today at 6:08 PM
It's not always trivial to saturate available memory bandwidth. You may need to use more than one core, or you need to carefully consider the topology of the chip you're working with (see lstopo). For example, what if the L3 cache is actually split in two [0]? In such cases, adding more bandwidth doesn't do much for you.
[0] https://en.algorithmica.org/hpc/cpu-cache/sharing/#saturatin...
today at 6:27 PM
Nine women cannot make a baby in one month.
today at 6:51 PM
128 cores x 288 pins per DIMM = 36864 pins on your socket, and all the motherboard traces. Good luck with that...
today at 5:40 PM
I was going to be sad than Zen 6c was divergent, but they increased a bunch of OoO structures on Zen 6c? To absorb more potential latency, since the memory is going to be heavier loaded? Ok, that I can appreciate!
It's absolutely wild that adjusting some l3 and using a smaller node goes from 12 to 32 cores for basically the same chip.
I wonder if the CCD will remain as monolithic as it does, for how long? The l3 latency being signficantly higher for 6c (10.3ns zen5 -> 10.7ns z6 -> 16.4ns z6c) feels like a symptom of a broader scaling challenge as the CCD hosts more and more. I can imagine dividing zen 7 or zen8 CCD into partitions, perhaps?
today at 8:21 PM
I think it's less there because of differences in the memory subsystem, but more because clock targets are lower so they can afford more. The structures they grew cost basically nothing in die area, but make timing tighter. It's possible they'd like them to be bigger for the non-c core too, but can't fit them in their timing constraints.
The unified L3 per CCD really helps on some key desktop loads (especially games). Most server throughput loads partition nicely into smaller partitions, but most games are hurt so bad by the latency penalty of crossing into a different partition that AMD's game drivers park the second CCD on their 2-CCD CPUs when they detect a game running. So long as games are an important target for them, I suspect they will not partition the CCD again. (It, of course, used to be parititioned for Zen 1 and 2.)
today at 6:10 PM
Rumor mill says they are going to use costlier interconnect to drive down CCD -> CCD latency in Zen 7+.
today at 6:23 PM
It is hilarious that the SPEC2026 graph does not include apple's P-cores. And no, not because they are ARM and not x86, since Graviton is included. It is because their integer & FP scores of 9.8 & 13 respectively absolutely wipe out everything else on this chart.
today at 7:50 PM
The single-threaded results of the Apple cores are excellent, but the multi-threaded results per socket of a Zen 6 absolutely wipe out any Apple CPU.
The Apple CPUs may be great for a personal computer, but they are not competitive with a server CPU.
Moreover, the SPEC floating-point results are not really representative for the floating-point performance of Zen 5 and Zen 6 that can be reached with optimized AVX-512 programs, which is much greater.
today at 6:55 PM
if you're interested in a fair comparison here's one https://openbenchmarking.org/test/pts/stockfish
today at 6:57 PM
Interesting, but seems to lack single-core benchmarks.
today at 7:38 PM
So this benchmark lists the following:
> AMD Ryzen 9 9950X 16-Core 63rd 4 40132379 +/- 2530724
> Apple M5 Pro 52nd 5 22108587 +/- 260160
I have absolutely no idea what this is benchmarking or how, but is there any reason not to just read this as the "consumer/low core ryzen" is twice as fast as the m5 pro?
today at 8:32 PM
Stockfish parallelizes well and makes heavy use of vector operations, and the Zen5 core has AVX-512 with 4x 512bit° EU per core and 16 cores, while the Apple still only uses 128-bit wide SIMD, with the 6 M5 "Super" cores having 4x128 EUs and the 12 "Performance" cores only having 2x128 EUs.
The M5 Pro is doing shockingly well given the massive disparity in execution resources. But still, it's optimized for web/desktop-like loads, you shouldn't really run heavy number crunching on that CPU.
0: 2x FMA/MUL + 2X ADD
today at 9:56 PM
Is there any sane way to generalize these results?
today at 7:39 PM
> I have absolutely no idea what this is benchmarking or how
It's the best chess engine used by millions of people and part of SPEC2026, natively compiled
today at 7:11 PM
it's not year 2006 anymore
today at 8:59 PM
I’m sorry, I wasn’t aware that Amdahl's law has been cancelled. I will brush up on my computing theory.
today at 6:48 PM
1. Different market
2. What flags & compiler?
3. Can't run Linux on that M5/M6 can we?
today at 7:06 PM
Linux is now (barely) working on M5/M6.
today at 6:57 PM
1. so is Ryzen 9 9950X
today at 7:46 PM
Regular Ryzen parts are still reasonably relevant for server use! They support ECC, so there's nothing immediately disqualifying them - see the rebranded "EPYC" 4005 series, for example.
A company like Hetzner isn't going to care an awful lot whether they deploy one big SP5 server or four small AM5 ones for the VM instances they rent out: the only thing that matters is TCO per performance.
Want to stick four NVMe SSDs, a 25Gbit NIC, 256GB of ECC RAM, and some OOB module on a Ryzen board? No problem whatsoever. Want to do the same with an Apple CPU? Ehhh, that's going to be slightly more challenging...
today at 6:59 PM
1. 9950X isn't on the chart.
The 9950X3D2 was already tested. And was included as comparison point with the fastest possible version of its predecessor (Zen 5).
Some evidence that contextless numbers are meaningless is that your source, David Huang, does include the 9950X but in his results it scores much higher than everything on C&C's chart too (7.25/11.4)
today at 6:33 PM
I mean, this is a review of the server grade parts. Apple is not exactly selling m chips to hyperscalers right?
Different story when you're talking about the context of a desktop or mobile.
today at 6:44 PM
The same graph has a bar for the Ryzen 9 9950X, though.
today at 9:09 PM
Ryzen 9950X is basically the same as an Epyc 4565P, except you can buy the Ryzen in a retail box.
today at 8:18 PM
Waxes nostalgic over the G5 Xserve